The technical sector concerned by this invention is that of sandwich panels resistant to impact, flames and agents attacking organic composite materials and is designed for making multilayer walls or partitions.
The sandwich panels are used to make structures combining high stiffness, low weight and possibly high thermal insulation, as well as being self-extinguishing. They have many applications: building, aeronautics, space industry, shipbuilding, transportation, armament, etc. In addition, the constituent materials can be: metals (aluminum, titanium, steel, etc.), minerals (cellular concrete, plaster, expanded or not, etc.) or plastics (resins, organic composites reinforced by fibers, etc.).
In all cases, the sandwich panel is the result of assembly of two skins, one external and one internal, made of materials with high mechanical strength and a high deformability modulus and a low density core acting as a spacer between the panels.
The skins are made from sheet metal (Al, Ti, steel, etc.) or laminates made of resin+fibers (epoxy, polyester, phenolic, vinylester, thermoplastic resins associated with glass, carbon, aramide fibers) or extruded and/or calendered sheets (thermoplastic or other), or boards of wood (plywood, etc.) or the association of several of the above materials (for instance aluminum sheet and plywood).
The cores are divided into three major families of materials:
expanded or lightweight materials such as plastic foams, concretes, etc.
wood (balsam being the typical example),
cellular materials such a honeycomb, tube spacers or self-stiffened ribbed plates, etc. To the honeycombs already known (aluminum, aramide, kraft paper, etc.) a new family of olefin thermoplastic materials insensitive to corrosion and ageing has recently been added. This family offers mechanical properties which are approximately equivalent to those of known honeycomb materials but at substantially lower costs. It has not yet been used in structural sandwich materials because it cannot be bonded or laminated to the standard skins. In addition, these olefin thermoplastic honeycombs raise problems of direct bonding with the thin metal plates.
As for the process for manufacturing the sandwich panels, it is characterized by the type of interface between the skins and the core.
There are three main ways of obtaining sandwich materials:
interface by bonding, sealing or mechanical assembly between each skin panel and the core,
production of the skin panel directly in contact with the core by molding or contact lamination (e.g. concrete, resin, etc.),
expansion of the core poured in situ between the two skin panels (e.g. expanded plaster, foams, etc.).
Considering the diversity of the constituent products mentioned above, it is possible to vary the properties of the sandwich panel by preferentially combining the materials according to their most advantageous properties.
Until now, the constructions and structures consisting of elementary panels were made of various materials, alone or in association, with major drawbacks. In effect:
thin metal sheets are deformed by the least shock received perpendicular or not to their surface; in addition, they are not easy to repair and do not provide thermal insulation,
laminates do not withstand buckling well; they do not form a firewall and the thermal insulation they provide is relatively small,
thermoplastic plates are not capable of withstanding temperature rises or flames for acceptable weights and costs per unit area,
honeycombs and other tubular spacers are not thermally insulating,
synthetic foams lose their mechanical properties when the temperature to which they are subjected rises above 80.degree. to 100.degree. C.
For guidance, reference can be made to patent EP-A-Nos. 155 335 and 111 520.
The production of a sandwich panel therefore requires the use of a number of materials in layers which must be assembled, generally by bonding or lamination.
It is known how to assemble a thermoplastic honeycomb and a woven or nonwoven fiber sheet. To do so, the base of the thermoplastic honeycomb cells is heated and the material thus softened is forced to penetrate through the fibers of the sheet. This is achieved industrially by feeding a thermoplastic honeycomb plate placed on the width of the sheet between heating rollers with suitably adjusted speed, separation and temperature. When cooled, mechanical assembly is achieved at the base of the honeycomb cells by catching of the thermoplastic resin which, when heated to semisolid state, penetrates the interstices of the sheet. This technique does not contribute any particular property to the honeycomb except mechanical assembly of its faces which provides flat surfaces on either side of the honeycomb to facilitate future interfaces.
In addition, it is very often useful to insert a metal sheet in a sandwich material to improve its thermal insulation and ageing resistance properties. Currently, it is not known how to bond a bare metal sheet directly on an olefin thermoplastic honeycomb.